1. Field of the Invention
The present invention relates generally to a height control system in an automotive suspension system for adjusting or regulating the vehicular height to maintain within a predetermined height range. More specifically, the invention relates to a vehicular height control system which can perform height adjustment even when both of drive wheel and power train are locked.
2. Description of the Background Art
In the recent years, there have been developed various vehicular height control systems for regulating height of a vehicle body relative to road surface and whwreby for regulating vehicular attitude. In the typical construction of prior proposed vehicular height control system, vehicular height is monitored by a vehicle height sensor which monitors height of the vehicle body relative to the road surface or to a suspension member rotatably supporting a road wheel. Height adjustment is performed by adjusting suspension force to be exerted between the vehicle body and the suspension member to maintain the vehicular height within a predetermined height range. In practice, the suspension force is generated by means of a pressure chamber which is connected to a pressurized fluid source to introduce thereinto and discharge therefrom a pressurized working fluid, such as air, viscous oil and so forth.
In such vehicular height control system, suspension member is pivotally moved about its pivot during height adjustment for varying vehicle body height. This pivotal movement of the suspension member causes rotation of the road wheel. Therefore, in case of the road wheel is a drive wheel to be driven by an engine output through a power train, a drive shaft connecting a differential gear unit of the power train to the road wheel for distribution of the engine output torque, has to be rotated. In other words, unless causing rotation of the road wheel and drive shaft, height adjustment at the position of the drive wheel cannot be performed.
When height adjustment command is generated while the drive wheel is locked and power train is locked which tends to occur when an automatic power transmission operation mode is selected at parking position, suspension member is restricted pivotal movement so as not to cause height variation of the vehicle. However, due to presence of the height adjustment command, fluid pressure in the pressure chamber is continuously increased or decreased for adjusting suspension force. If lock on the drive wheel and the power train is released at this position, the drive wheel and the drive shaft becomes free from restriction and thus can rotate. This causes sudden change of vehicle height. This clearly degrades riding comfort of the vehicle.
Furthermore, when the drive wheel and power train is maintained for relatively long period of time, adjustment of the suspension force tends to overshoot to cause hunting in height adjustment. Namely, assuming the height adjustment command generated in the condition wherein the drive wheel and the power train are locked is DOWN command, the fluid pressure in the pressure chamber can be reduced excessively. Therefore, when lock of the drive wheel and the power train is released, the vehicle height drops across the lower limit of the predetermined height range to alternatively cause UP command. This subsequently cause height adjustment in UP direction. Such action tends to cause uncomfortable rid feeling for the vehicular occupants.